Review of the Prior Art and Its Problems
Many ingenious and sophisticated proposals have been made in the field of ocean engineering which call for the use of large diameter vertical ducts of great length and which extend from at or near the ocean surface to lower ends unconnected to the ocean floor. These proposals include concepts for ocean thermal energy conversion and for mariculture.
The ocean thermal energy conversion concepts propose to use the difference in thermal energy levels between warm surface water and colder deep water to generate electricity, for example. The available energy level difference is low, and so these proposals rely on the use of very large quantities of warm and cold water, and call for the necessary large volumes of deep ocean cold water to be brought to the water surface through very large vertical ducts of great length.
The mariculture proposals typically call for large quantities of cold nutrient-rich deep ocean water to be brought to the warmer surface portions of the ocean to provide food for the growth of ocean plants or animals. For example, one proposal calls for kelp to be grown on frames located about 60 feet or so below the ocean surface and to be nourished by nutrient-rich water brought up to the vicinity of the frames from 1500 feet or more below the ocean surface.
These proposals, and others similar to them, have the common feature of requiring the use of very long, large diameter vertical ducts through which water from deep in the ocean may flow upwardly. The rate of water flow through the ducts, called upwelling ducts, would be relatively low, and thus the pressure differential across the walls of the duct would be correspondingly low, especially if the water in the duct is not carried far above the ocean surface before being discharged from the duct.
The approaches suggested to date for implementing these proposals have described the water upswelling ducts in conventional terms. Thus, these large diameter water upwelling ducts have been described in the various proposals as being stiff pipes, albeit of large diameter, suspended from upper ends supported on suitable platforms, such as floating or submerged buoyant platforms.
Any structure which extends vertically for any significant distance in the ocean will encounter at least one ocean current. Currents impose drag forces upon such structures. The larger the structure, the greater its profile (effective area) presented to the current, and therefore the greater the drag forces which a given current will impose on the structure. These drag forces impose bending loads on the structure, and the longer the structure, the more severe the resulting bending stresses and deflections.
The problem of ocean current drag forces and of the bending stresses produced by such forces is a significant difficulty in the offshore drilling industry in regard to the riser pipes which are used to conduct drilling mud, during the drilling of a subsea oil or gas well, from the well bore to the surface drilling platform. Current induced bending stresses are so great in these riser pipes that the pipes must be made very heavy, i.e., with thick walls, to enable them to withstand the bending stresses. This problem is such that, because of it, offshore drilling operations today are effectively limited to water depths of about 1000 feet or so. This limit is imposed principally by the riser pipes.
The riser pipes now in use in 1000 foot water depths by the offshore drilling industry are relatively large in diameter, but they are very much smaller than the diameter of the much larger upwelling ducts called for in the various ocean engineering proposals reviewed above. The very large, very long upwelling ducts proposed would be subjected to very large drag forces and very high levels of bending stress. If upwelling ducts of conventional description (stiff, continuous pipes and the like) are to be used, they must be extraordinarily heavy, thickwalled and costly, and very difficult and expensive to assemble and put into place. The problems associated with these upwelling ducts are a major impediment to the economic implementation of the proposals for development of mariculture and ocean thermal energy conversion.
A need is therefore seen to exist for novel large diameter duct structures of great length adapted to be suspended in an ocean, and yet be capable of withstanding or accommodating current-induced drag loads and the effects of such loads.